1. Field of the Invention
The present invention relates to a conical horn antenna applied to, for example, a primary horn of a reflector antenna or an electromagnetic horn antenna itself. More particularly, the invention is concerned with a conical horn antenna (hereinafter referred to as "plural mode horn antenna") used for and excited by plural modes of electromagnetic waves, including an electromagnetic wave of TE.sub.11 mode which is a dominant mode of a circular waveguide constituting a conical horn and higher modes of electromagnetic waves.
2. Description of the Prior Art
The horn antenna is known as an antenna capable of affording relatively high gain in spite of the simple structure, and various attempts have been made to improve the radiation characteristic of the horn. Moreover, a horn which has been improved to obtain a high gain is often used as a primary radiator of another aperture antenna.
FIGS. 1 and 2 are schematic sectional views showing examples of conventional plural mode horn antennas disclosed in Japanese Patent Publication No. 9047/81.
The plural mode horn antenna shown in FIG. 1 is composed of a feed waveguide 1 for feeding an electromagnetic wave of the basic TE.sub.11 mode, a desired mode of electromagnetic wave generating portion 2 connected to the feed waveguide 1 for generating a magnetic transversal wave of TM.sub.11 mode, and a truncated cone-like conical horn 3 connected to the desired mode of electromagnetic wave generating portion 2. The desired mode generating portion 2 is composed of a tapered waveguide 4 expanding from the feed waveguide 1 toward the conical horn 3 and a straight cylindrical waveguide 5 which connects the tapered waveguide 4 and the conical horn 3 with each other. Such a shape of a conical horn antenna is called a flare type plural mode horn antenna in view of its tapered shape.
The operation of the conventional plural mode horn antenna of the above construction will now be described. A portion of the electromagnetic wave of TE.sub.11 mode as the dominant mode fed from the feed waveguide 1 is converted to an electromagnetic wave of TM.sub.11 mode as a desired mode at the discontinuous portions of the connection of the tapered waveguide 4 and the straight cylindrical waveguide 5 and of the connection of the waveguide 5 and the conical horn 3. The size of the conical horn 3 is determined by obtaining a desired radiation pattern, while the inside diameter of the straight cylindrical waveguide 5 is determined by diminishing the occurrence of unnecessary higher modes. Therefore, a desired amount of electromagnetic wave of TM.sub.11 mode to be generated and that of phase thereof can be obtained by selecting an optimum relation between the length of the straight cylindrical portion of the straight cylindrical waveguide 5 and the expansion angle of the tapered waveguide 4. Further, a radiation pattern reduced in the proportion of a cross polarization component can be obtained by adopting a shape which is symmetric rotationally.
However, in the above so-called flare type plural mode horn antenna having the tapered waveguide 4, it has been impossible to obtain good radiation characteristics in a wide frequency band.
In an effort to overcome the above problem, there has been proposed a so-called flare iris type plural mode horn antenna, as shown in FIG. 2, in which in addition to the tapered waveguide 4, iris diaphragms (hereinafter referred to simply as "irises") 6 as circumferential projections are formed in plural stages on the inner peripheral surface of the straight cylindrical waveguide 5. In the example shown in FIG. 2, the above irises 6 comprise a first iris 6a and a second iris 6b which are arranged in two stages. The operation of this flare iris type plural mode horn antenna will now be described. Like the foregoing antenna show in FIG. 1, an electromagnetic wave fed through the discontinuous shape from the tapered waveguide 4 to the conical horn 3 via the straight cylindrical waveguide 5 is converted to the electromagnetic wave of TM.sub.11 mode, and also from irises 6 there is generated the electromagnetic wave of TM.sub.11 mode, which wave is propagated toward the conical horn 3 and also toward the tapered waveguide 4, namely, in the flare direction. The electromagnetic wave in the direction of the tapered waveguide 4 is reflected by the tapered inner surface and transmitted toward the conical horn 3. Therefore, by suitably selecting the sizes of the tapered waveguide 4, straight cylindrical waveguide 5 and irises 6 there can be ottained an electromagnetic wave of TM.sub.11 mode over a wide frequency band in a desired amount thereof generated and that of phase thereof.
Further, there has also been proposed a desired mode of electromagnetic wave generating portion 2 in which the flare portion and the straight cylindrical waveguide portion formed with iris in the flare iris type plural mode horn antenna of FIG. 2 are provided in plural stages as shown in FIG. 3. This plural mode horn antenna illustrated in the schematic sectional view of FIG. 3 is disclosed in Japanese Patent Publication No. 17164/85. In the said figure, the desired mode of electromagnetic wave generating portion 2 is composed of a first tapered waveguide 4, a first straight cylindrical waveguide 5, a second tapered waveguide 7 and a second straight cylindrical waveguide 8. On the inner peripheral srrface of the first straight cylindrical waveguide 5 are formed irises 6a and 6b as circumferential projections, and the second straight cylindrical waveguide 8 is also formed with irises 6c and 6d. The irises 6a to 6d are axially provided metallic bands which are symmetric rotationally with respect to the axes of the first and second straight cylindrical waveguides 5 and 8. The inside diameter of the first straight cylindrical waveguide 5 is set at a size which permits transmission of an electromagnetic wave of TE.sub.11 mode but does not permit transmission of TE.sub.12 mode electromagnetic wave, while the inside diameter of the second straight cylindrical waveguide 8 is set at a size permitting transmission of the TE.sub.12 mode electromagnetic wave.
In the plural mode horn antenna of the above construction, an electromagnetic wave of TM.sub.11 mode is generated by both end portions of the first straight cylindrical waveguide 5 and discontinuous portions formed by the irises 6a and 6b. Further, electromagnetic waves of TM.sub.11 and TM.sub.12 modes are generated by both end portions of the second straight cylindrical waveguide 8 and discontinuous portions formed by the irises 6c and 6d. By controlling these two higher modes there are obtained good electromagnetic wave radiation characteristics over a wide range of frequency band.
FIG. 4 is a schematic sectional view of a further conventional plural mode horn antenna which is disclosed, for example, in Japanese Patent Laid Open No. 204604/83. In the horn antenna shown therein, a desired mode of electromagnetic wave generating portion 2 comprises first and second tapered waveguides 4 and 7 as well as first and second straight cylindrical waveguides 5 and 8, which are connected in an alternate fashion. A first feed waveguide 1 feeds electromagnetic waves belonging to a high frequency band. To the second tapered waveguide 7 is connected a second feed waveguide 9. In the illustrated example, two feed waveguides 9a and 9b are open to the tapered surface in opposed positions. These second feed waveguides 9a and 9b feed electromagnetic waves belonging to a low frequency band.
The plural mode horn antenna constructed as above is used in both low and high frequency bands. The inside diameter of the first straight cylindrical waveguide 5 is set at a size which permits transmission of only electromagnetic waves of TE.sub.11 and TM.sub.11 modes belonging to the high frequency band, not permitting transmission of TE.sub.12 mode electromagnetic wave, and which prevents the electromagnetic waves in the low frequency band fed from the second feed waveguides 9a and 9b attached to the second tapered waveguide 7, from being transmitted in the direction of the first straight cylindrical waveguide 5. The inside diameter of the second straight cylindrical waveguide 8 is set at a size which permits transmission of elecrromagnetic waves of not only TE.sub.11 and TM.sub.11 modes in the high frequency band but also the same modes in the low frequency band. Thus, in the plural mode horn antenna shown in FIG. 4, the first tapered waveguide 4 and the straight cylindrical waveguide 5 can be designed in conformity with the high frequency band, and the second tapered waveguide 7 and the straight cylindrical waveguide 8 in conformity with the low frequency band. Consequently, good electromagnetic wave radiation characteristics can be obtained in a wide frequency range including both frequency bands.
In addition to the above four examples of conventional plural mode horn antennas, there are the following several examples of prior art literatures described in English:
(1) "A New Antenna with Suppressed Sidelobes and Equal Beamwidths", written by P. D. Potter, reprinted from MICROWAVE JOURNAL, June, 1963.
(2) "Flare-Angle Changes in a Horn as a Means of Pattern Control" written by DR. SEYMOUR B. COHN, the Microwave Journal, October, 1970.
(3) "Conversion of TE.sub.11 Mode by Large Diameter Conical Junction", IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, MAY, 1969.
The conventional plural mode horn antennas described above using FIGS. 1 to 4 have involved the following problems.
First, in the conventional examples shown in FIGS. 1 and 2, a large expansion angle of the tapered waveguide 4 results in poor matching with the feed waveguide 1 and deterioration of the standing wave ratio because the angle of the tapered waveguide 4 of the desired mode of electromagnetic wave generating portion connected to the feed waveguide 1 is determined to obtain a desired mmount of an electromagnetic wave of a desired mode, e.g. TM.sub.11 mode, to be generated and that of phase thereof.
There has also been the problem that if the expansion angle of the tapered waveguide 4 is set small in order to improve the standing wave ratio, the aforementioned amounts in the desired mode cannot be obtained.
In the plural mode horn antenna shown in FIG. 3, moreover, since electrically it is necessary to generate electromagnetic waves of TM.sub.11 and TE.sub.12 modes in addition to the dominant mode and control the three modes simultaneously, it has been difficult to design an antenna having good radiation characteristics over a wide frequency range including a low frequency band, e.g. from 12.25 GHz to 12.75 GHz, and a high frequency band, e.g. from 14.0 GHz to 14.5 GHz. Also mechanically there has been the problem that the production is difficult because it is necessary to form annular and band-like irises 6 . . . on the inner peripheral surfaces of the first and second straight cylindrical waveguides 5 and 7.
Further, in the plural mode horn antenna shown in FIG. 4, there have been the following problems electrically and mechanically although good radiation characteristics can be obtained in a wide frequency range including low and high frequency bands. First, electrically, an electromagnetic wave of a low frequency band is prevented from being transmitted through the first straight cylindrical waveguide 5, while allowing an electromagnetic wave of a high frequency band, e.g. TM.sub.11 mode electromagnetic wave, to pass therethrough, so the low and high frequency bands are restricted in the frequency ratio. Mechanically, moreover, there has been the problem that the structure of the horn antenna becomes complicated because it is necessary to separately provide the second feed waveguide 9 which communicates with the inner tapered surface of the second tapered waveguide 7 and which is for feeding electromagnetic waves belonging to the low frequency band.